Overview

CVA (Ceiling and Visibility Analysis) is a real-time analysis of current C&V
(ceiling and visibility) conditions across the continental U.S. The product is
primarily intended to help the general aviation pilot (particularly the VFR-only
pilot) avoid IFR conditions. However, CVA's quick-glance overview of ceiling and
visibility conditions can be useful to others involved in flight planning or weather
briefing. To remain current in rapidly changing conditions, CVA is updated every
five minutes. The overview provided by CVA should be followed by further examination
of METARs, TAFs, AIRMETs, Area Forecasts and other weather information.

Inside CVA - How It Works

The product makes use of real-time METAR observations to establish
C&V (Ceiling & Visibility) conditions at approximately 2000 observing
points.

Ceiling

At display points between METAR locations (where there are no direct
observations), CVA ceiling values are derived as follows:

First, nearest-neighbor interpolation is used to find an initial estimate
of ceiling height. This initial estimate is the ceiling height at the nearest
METAR location.

Next, the initial height is corrected for any change in surface
elevation between the METAR location and the CVA display point. For example,
if terrain at the display point is 200 feet higher than at the METAR site,
the ceiling value (given as feet above ground level) is taken as 200 feet
less than the value at the nearest METAR location. If the resulting ceiling
height is negative, a value of zero is substituted.

Finally, a clear-cloudy test using GOES (Geostationary Operational
Environment Satellite) satellite data is applied. If a conservative
interpretation of real-time GOES data indicates that the display point is
cloud free, then the ceiling height at that point is reassigned as "unlimited".

Visibility

Visibility values at display points are taken as the visibility observed at
the nearest METAR site and are displayed as (i) less than 3 miles
(pale yellow), and (ii) 3 miles or greater (pale blue).

Flight Category

Flight category values are derived from ceiling and visibility display
values according to the official definition, as given in the table below.

While CVA provides an accurate representation of observed conditions at
METAR locations, only estimated conditions are available at regions between
METAR sites, and these estimates are subject to error.

NOAA carried out a study comparing CVA ceiling and visibility estimates
with actual observations. Understanding the results of the study can
improve a user's understanding of how to best use the CVA product. The study
is entitled, NCVA_D4_SUB_REGION_ENHANCEMENT_FINAL.pdf

Access to CVA Grids

CVA grids are made available by the NWS (National Weather Service)
as GRIB2 (GRIdded Binary) files for distribution and archive via NOAAPORT
(for subscribers) and via FTP (File Transfer Protocol) from the NWSTG
(National Weather Service Telecommunications Gateway).

Disclaimer and Warning

By FAA policy, CVA is a Supplementary Weather Product for enhanced situational
awareness only. CVA must only be used with primary products such as METARs, TAFs
and AIRMETs.

See the Supplementary Weather Product Description page for a definition of these terms.
Also see AIM Section 7-1-3
for requirements regarding use of aviation weather products.

Cautionary Note to CVA Users

The skill of METAR and satellite observations declines with distance from the
nearest METAR station, and in variable terrain. VFR pilots and IFR pilots who are
not fully proficient and fully equipped for flight under IFR conditions should use
increasing caution as distance from the nearest METAR station increases and in areas
of variable terrain.

CVA is a current real-time analysis only. It does not represent a forecast,
nor can it be used to infer a forecast.